This invention describes a composition of matter having moisture gettering properties and applicable as coating or adhesive in a hermetic microelectronic device.
Modern packagings used for electronic devices, such as integrated microcircuits (silicon chips or dies), hybrid microcircuits and surface mounted devices used as transistors, diodes, resistors, capacitors, transducers and such, are designed both to protect the sensitive components and circuits mechanically and environmentally, and to provide a functional interface to macroscopic application, such as printed circuit board. Such packagings are usually constructed from metals or ceramics and hermetically sealed in a moisture free atmosphere to minimize the possibility of corrosion during usage. Hermetic seals may be formed by soldering, welding and sealing through glass.
There has been continuing concern regarding the presence of water vapor in hermetically sealed semiconductor devices. This concern is based upon both theoretically possible failure modes and actual observation of failure caused by corrosion due to the presence of moisture.
Hermetic microelectronic devices used in military, space and other applications requiring high reliability have an upper limit of 5,000 parts per million (ppm, by volume), of water vapor content at the time of fabrication. Package leak rate is limited to 10.sup.-8 atm - cc/sec. maximum to prevent leakage of a significant amount moist ambient air into the packaging during the device's useful lifetime. In spite of extreme precaution, it is very difficult to manufacture a hermetic packaging for microelectronic devices with low water vapor content and to maintain it during its useful lifetime. There are various channels water vapor finds its way to the inside of the enclosure:
1. The various seals in a packaging are usually not perfect and it has a small but enough leak to let ambient air, containing moisture, inside the enclosure. PA0 2. Many epoxies used to bond dies and substrates outgas moisture in the packaging with time. PA0 3. The packaging material itself outgasses a certain amount of moisture. Prebaking prior to sealing may not liberate all the absorbed moisture. PA0 4. The sealing atmosphere may be contaminated with moisture. PA0 5. The testing process itself for leak rate may introduce moisture inside the package if not done with extreme care.
The prior art tried to solve the moisture problem by the application of a moisture barrier coating, also referred to as passivating layer, on the microcircuit. Such products and processes include total passivation with silicone compounds and surface passivation with silicon oxide, silicon nitride and plasma deposition of polymerized hexamethyldisilazane. U.S. Pat. Nos. 4,163,072, 4,059,708 and 4,628,006, issued, in order, to Soos, Heiss, Jr. et al. and Schnessler are illustrative of such methods. However, for a number of practical reasons these techniques were found to be less than adequate.